> Sulfur is another (different crystal types; all with mp = roughly 100°C)What's your friggen' point? sulfur doesn't resemble methamphetamine at all. so let's compare to a corresponding salt:
S: ~100°C
Na2S: >900°CSee? your example impressively supports my statement saying that salts generally have higher melting points than non-salts.
> Hypo, maybe you could explain at this point why elementary sulfur (for example) does form crystals?Erm, please reread my posts above: there are different types of bonds: some bring more energy, some less.
Many weak bonds can bring as much energy as one strong bond and note how in the case of amphetamine there's a rather big non-polar molecule around the ionic bond. lets remove this non-polar thingy and we get:
NH4Cl: 340-341°CSurprise, surprise, the melting point increases!
Let me recapitulate:
Q: why do many of our beloved amine freebases exist as oil at STP conditions whereas the corresponding salts are practically always solids.
A: Whether at given conditions the thermodynamically favored form of a substance is a liquid or a solid depends on two factors:
1) Formation of solids means a reduction of entropy (molecules are restricted in their movement and there is formation of an ordered structure).
2) Formation of solids liberates bonding energy.
If the energy gained by the new bonds outweighs the energy corresponding to the loss of entropy, then the thermodynamically favored form will be a solid. since on one side there are only weak bonds between the freebase molecules and on the other side the salts form very strong ionic bonds, the former will often not solidify at STP, whereas the latter almost always will.
(Of course if one is bored one could imagine an exception to this rule, where for example the amine functionality only makes up for a small part of the molecule and for whatever reason the formation of the corresponding salt breaks the crystal structure in such a way that the salt melts at a lower temperature than the freebase, but this would be a very esoteric case, irrelevant for our purposes.)
Alright, someone who doesn't get it now will probably never get it, so i rest my case.